Patent GB-496825-A (GB)
Title: Methods of and apparatus for measuring electrical capacity and other magnitudes
IPC: G01R27/26
Filed: 1937-07-30
Abstract: 496,825. Inductance, capacitance, and voltage measurements. MURPHY RADIO, Ltd., BARTLEY, W. B., TOMES, S. R., and WHITE, E. L. July 30, 1937, No. 21178. [Class 37] In apparatus for comparing reactances and oscillation generators, the output of a frequency changer fed from two oscillators containing or constituting the component to be tested and the standard is fed to a frequency discriminating network which produces a direct voltage varying with the frequency of the frequency changer output. This voltage is amplified and measured by a D.C. milliammeter. A condenser 1, Fig. 1, under test forms part of the oscillatory anode circuit 1 ... 8 of a valve 9, the anode circuit being coupled to a grid circuit 11 ... 14 to form an oscillation generator. Similarly, a standard variable condenser 15 is included in the oscillatory anode circuit 15 ... 18 of a valve 19, which has a coupled grid circuit 21 ... 24. These two oscillators feed, through series-connected coupling coils 25, 26, condenser 27 and grid leak 28, the grid of a frequency changer 29. The beat frequency is supplied to a discriminating network, the high-frequency components being byepassed by a resistance 38'through a condenser 39. The discriminating network comprises two oscillatory circuits 31, 32, fed through small condensers 33 and supplying rectifiers 34, 35 connected by condenser-shunted load resistances 36, 37. The circuits 31, 32 are tuned to frequencies equally above and below the normal beat frequency of the frequency changer output. The D.C. voltage produced across the resistances 36, 37 is applied to the grid of an amplifying valve 42 through a filter circuit 43 and is measured by means of a D.C. milliammeter 41. The valve 42 has an adjustable bias resistor 44 in its cathode circuit and a fraction of its anode voltage is applied by the potentiometer 45 through a resistance 46 to the meter. The milliammeter 41 may have markings indicating the permissible limits of capacitance of the condenser 1 under test, or capacitance may be added by variable condensers 3, 4 to the circuit of either the condenser under test or of the standard condenser until the meter reading is brought to normal. The attenuator 38, 39 may be replaced by a low-pass filter consisting of series inductances 48, 49, Fig. 2, and shunt capacitances 51, 52, 53. A filter consisting of condensers 54, 55, an inductance 56, and a resistance 57 may be inserted between one or both oscillators 9, 19 and the frequency changer 29 in order to cause the meter to indicate percentage error or actual error throughout the range of the apparatus. A triode may be substituted for the pentode 29. Where the instrument is to indicate percentage error directly the valve voltmeter shown in Fig. 3 may be employed. Two triodes 58, 59 have their cathodes connected together and to a common bias resistor 61, a milliammeter 41 being connected between their anodes. The outer ends of the resistances 36, 37 are connected to the grids of the valves 58, 59 through decoupling resistances 63, 64, resistances 65, 66 serving to equalize the voltages applied to the two grids. One of the anode resistances 67, 68 is adjustable in order to bring the ammeter to its mid-reading when no beat frequency is applied to the discriminator. The supply to the oscillators and frequency changer may be stabilized by a neon tube. Where the apparatus is used for testing a variable condenser by comparison with a standard variable condenser, the condenser to be tested may be mounted in a magnetic chuck and its spindle coupled to that of the standard condenser. Inductances and variable inductances may be tested similarly, or the whole of the oscillator on the test side may be the subject of test, the apparatus serving to compare its frequency with a standard.